1. Field of the Invention
The present invention relates generally to a control system for controlling operation condition of an engine for a motor vehicle, and particularly to a control system for controlling a DC control motor which controls operation of an internal combustion engine to follow-up a target value varying continuously depending on the operation condition of the engine.
2. Prior Art Statement
During cruising of a motor vehicle, the rotation speed of the engine is changed greatly and the loading applied on the engine is also changed within a wide range. In consideration of the foregoing, it has been proposed to detect the operation conditions of the engine and to open or closed a variety of control valves depending on the result of detection so as to achieve optimum control of the engine. For example, Japanese Patent Laid-Open Publication No. 126222/1987 teaches a system wherein an exhaust gas control valve is disposed at a vicinity of the downstream end of the exhaust pipe, and the exhaust gas control valve is fully opened to utilize the kinetic effect of the exhaust system at the maximum extent so as to increase the output of the engine when the engine is operated within its high speed range. The exhaust gas control valve is closed to about one half of the full open angle, when the engine is operated within its medium speed range, to prevent formation of trough of torque due to the reverse effect of kinetics in the exhaust system. Another proposal has been made to change the effective length of the exhaust pipe by the provision of control valves on the connection pipes connecting the plural exhaust pipes and by opening or closing the control valves depending on the change in rotation speed of the engine.
When such a control valve is opened and closed by means of a DC motor which is controlled through pulse width modulation system (hereinafter referred to as "PWM system"), the duty ratio of the current flowing through the DC motor is controlled depending on the difference between the target value and the up-to-date value and/or depending on the change in loading applied on the DC motor. However, in the event where carbon or dust sticks to the control valve to increase the loading applied on the DC motor, the motor is operated at a higher duty ratio for a long time, leading to a result that the current flowing through the DC motor becomes excessively high to raise the temperature of the motor and eventually to cause burn-out of the motor.
On the contrary, in the event where the difference between the target value and the up-to-date value is relatively small and the duty ratio is also small and where carbon or dust sticks to the control valve to increase the torque for actuating the control valve, the control valve cannot be moved by he motor to be kept stopping.